seccomp: add forgotten munmap() syscall to @file-system (#5291)

We added mmap() and mmap2(), but forgot munmap(). Fix that.

Pointed out by @lucaswerkmeister:

https://github.com/systemd/systemd/pull/4537#issuecomment-273275298

src/shared/seccomp-util.c

@@ -344,6 +344,7 @@ const SyscallFilterSet syscall_filter_sets[_SYSCALL_FILTER_SET_MAX] = {
                 "mknodat\0"
                 "mmap2\0"
                 "mmap\0"
+                "munmap\0"
                 "newfstatat\0"
                 "open\0"
                 "openat\0"